The present invention relates to a cold-rolled steel sheet with properties of high-strength, high-ductility, and a process for manufacturing the cold-rolled steel sheet. Particularly, the present invention relates to the cold-rolled steel sheet having chemical compositions of 0.08%-0.25% carbon by weight, 0.3%-2.0% silicon by weight, 0.6%-1.8% manganese by weight, 0.01%-0.10% niobium by weight, 0.01%-0.08% aluminium by weight, substantial iron, and unnoticed impurities, and a process for manufacturing it. The tensile strength TS of the cold-rolled steel sheet described above is more than 690 MPa, and the formability is excellent.
Recently, cold-rolled steel sheets of high-strength, tensile strengths of about 440 MPa-690 MPa, and thicknesses of about 0.8 mm-1.6 mm have been used in bumpers and side-doors of automobiles in consideration safety of car passenger and lighting the weight of automobiles so as to minimize the fuel consumption. Many kinds of cold-rolled steel sheets of high-strength have been continuously developed so far, such as solid-solution hardened steel, precipitation hardened steel, recovery-annealed steel, dual-phase steel, full martensitic steel, and multi-phase steel containing retained austenite. However, each kind has its drawbacks which are listed below: